In an MRI system it is desired to make so-called movies of a human heart as it's beating. Naturally, the "R" heart pulses are normally used as the triggering point for these images. Then, by a sequence of RF pulses within a period of the heart beat, a sequence in time of images is taken. In accordance with well known MRI technique, this must be repeated over several heart beats; for example, as many as 128 or more times.
Because the heart rate of each individual being examined is unique, the foregoing technique must be adjusted to accommodate such heart rate since the "R" wave is used as a triggering point. If a gap is left between the last RF pulse in the sequence of RF pulses and the first RF pulse of the subsequent sequence, there is a longer relaxation period at this point. Because the magnetization has more time to relax towards equilibrium during this longer relaxation period, the image generated by the first RF pulse in the sequence will be brighter or appear to flash, compared to the remaining sequence of pictures used to make up the so-called heartbeat movie. This one picture can be rescaled but if this is done the problem of an unequal signal to noise ratio remains. p Another solution to the so-called "flash" problem is to provide a continuous train of RF pulses throughout the heart period and then reorder the data as necessary to approximate a trigger on the "R" wave. This is complex and may not fully compensate for all variations of unequal relaxation time between RF pulses. cl OBJECT AND SUMMARY OF INVENTION
It is therefore a general object of the present invention to provide a method of optimizing flip angles in MRI for unequal delay times.
In accordance with the above object, there is provided a method of optimizing radio frequency (RF) flip angles in a magnetic resonance imaging (MRI) system where multiple sequential images in time are desired and the time between RF pulses may be unequal comprising the following steps. A mathematical relationship is determined relating the effect of flip angle in a sequence of RF pulses, with an unequal time between at least some of said pulses, to the magnitude of the RF output signals of the MRI system due to the RF pulses. Then the relationship is utilized to derive flip angles for each RF pulse of the sequence to provide output signals which are constrained to be equal in amplitude and maximized.